EP1645031B1 - Low-pass filter for the filtering of adsl signals on telephone lines - Google Patents

Abstract

The invention relates to a low-pass filter (500), for the filtering of ADSL signals, by the coupling of different inductances to a passive autotransformer (501), in an electronic circuit, comprising in particular, elliptical cells arranged in a cascade and, more particularly, in the non-equilibrated forms of said electronic circuit, compensating for a negative inductance induced by said inductive coupling. In certain embodiments of the invention, the coupling compensation is in the form of an active circuit which emulates the characteristics of an inductance whose value is sufficiently high to compensate for the induced negative inductance.

Description

The present invention relates to a low-pass filtering device involved in the decoupling of the ADSL channels. It is intended to be incorporated in a voice-data signal separator device for transmission over ADSL channels. It proposes a particular embodiment of filters, whose performances are comparable to those of elliptical filters, and whose size is less important than that imposed by the structure of said elliptical filters.

The filtering device according to the invention is intended to be used on the subscriber side or in telephone exchanges, which are points of distribution and grouping of telephone communications and data transmitted according to ADSL technology.

The field of the invention is, in general, that of the so-called ADSL technology (for Asymmetric Digital Subscriber Line, in English, for asymmetric rate digital subscriber line), it will likewise apply to other technologies. DSLs that require voice-data separation. This technology is essentially implemented on local loops of existing telephony networks, the local loop being constituted by the intermediaries of the network between a central office and a subscriber station. One of the essential objectives of this technology is to make possible, on the local loops of the old telephony networks, the simultaneous transmission of different types of signals: on the one hand, the signals relating to the usual telephone conversations between two subscribers, these first signals comprising voice signals and signaling signals, and data signals, typically information exchanged between a subscriber and a site accessible via the Internet.

For transmissions using this technology, the pair of copper wires that arrives at the subscriber's premises and which has long been the telephone line, or subscriber loop, to which the different telephone handsets of the user are connected is thus used. traditional analogue networks called POTS (for Plain Old Transmission System) or traditional digital networks called ISDN (or ISDN - digital integration network). services). This pair of copper wires is most often twisted and comprises an insulating sheath of polyethylene.

• en bande de base, des signaux analogiques non modulés correspondant aux réseaux POTS, ou des signaux numériques correspondant aux réseaux ISDN. Les signaux analogiques en bande de base sont de fréquence typiquement 0 à 4 kHz, ou 0 à 16 KHz si on tient compte des signaux de signalisation. Les signaux numériques ISDN en bande de base sont dans des fréquences de 0 à 94 KHz. En pratique, la bande de base est destinée à une transmission en mode circuit;
• en bande haute fréquence, d'autres signaux, numériques et modulés, qui correspondent par exemple aux informations échangées avec le réseau Internet. La bande haute s'étale du dessus de la bande de base téléphonique jusqu'à 1 MHz environ. En pratique la bande haute fréquence s'étale de 32 KHz à 1,1 MHz si la paire de fils de cuivre sert en bande de base pour une transmission analogique POTS, et de 138 KHz à 1,1 MHz si elle sert pour une transmission numérique. Elle est normalement destinée à une transmission permanente, en mode paquets principalement, c'est à dire qu'une communication permanente est établie entre l'abonné et le central téléphonique. On est ainsi dispensé des opérations classiques intervenant lors d'une connexion avec accès par ligne commutée, qui est une méthode de connexion temporaire à un réseau informatique consistant à utiliser un modem, un logiciel de connexion et le réseau téléphonique commuté comme moyen de mise en communication de son propre ordinateur et d'un autre ordinateur du réseau.

On this pair of copper wires, one transmits at the same time:

• in baseband, unmodulated analog signals corresponding to POTS networks, or digital signals corresponding to ISDN networks. Analog baseband signals typically have a frequency of 0 to 4 kHz, or 0 to 16 kHz if signaling signals are taken into account. ISDN digital baseband signals are in the frequency range 0 to 94 KHz. In practice, the baseband is intended for transmission in circuit mode;
• in the high frequency band, other signals, digital and modulated, which correspond for example to information exchanged with the Internet. The high band spreads from the top of the baseband to about 1 MHz. In practice, the high frequency band ranges from 32 KHz to 1.1 MHz if the pair of copper wires is used in baseband for analog POTS transmission, and from 138 KHz to 1.1 MHz if it is used for transmission. digital. It is normally intended for permanent transmission, mainly in packet mode, ie a permanent communication is established between the subscriber and the central office. This eliminates the traditional operations involved in a dial-up connection, which is a method of temporarily connecting to a computer network using a modem, a connection software and the switched telephone network as a means of implementation. communication of one's own computer and another computer on the network.

Although the baseband can be used, especially with digital preference modems, to transmit data, and the high frequency band can be used to provide phonic communications in the context of so-called telephone communications over the Internet, it will be considered that schematically, and for the sake of simplification of the explanations, that the baseband is used for the transmission of a first type of signals, essentially words, corresponding to phonic communications, and that the high frequency band serves for the transmission of a second type of signal, essentially data, relating in particular to the consultation of sites Internet.

The baseband is only capable of low bit rate (currently 56 Kbps for analog modems and 64 Kbps for digital modems) while the high frequency band is likely to have a high bit rate (which may be close to 10 Mbps).

The figure 1 illustrates the use of the frequency band in ADSL distribution. The ordinate axis 100 scales the power of the transmitted signals, and the abscissa axis 101 gives the frequency scale. The transmitted powers are represented equitably for the purpose of simplification. Voice spectrum 102 ranging from 0 to 4 kHz is found, and spectrum 103 used for ADSL data transmission, the latter ranging from about 30 kHz to 1.1 MHz. The spectrum 103 is divided into two main parts: a first frequency band 104 corresponding to the spectrum used for the upstream data (from the subscriber to the central office) and a second frequency band 105 corresponding to the spectrum used for the downstream data (the central to the subscriber).

The use of a so-called ADSL line therefore requires the separation of the low frequency baseband from the high frequency band. The figure 2 schematically illustrates these separations at a local communication loop. In this figure, there is shown a usual telephone line 200, or transmission line, or subscriber loop, composed of two copper wires, which provides the connection between a subscriber 201, which has a private installation, and a central office 202.

The central office 202 connects to a conventional telephony network 203, called POTS, and an Internet type network 204. In order to dissociate the signals intended for the telephone network 203 and the Internet network 204, a first device 205 signal separator voice-data, also called a switch or splitter filter, is used. In general, a splitter is an electronic device associated with an ADSL modem that separates the voice signals from the data signals and routes them to two different paths. The splitter 205 will be designated later as central splitter. The data destined for - or coming from - the Internet network 204 passes through a DSL access multiplexer 206, called DSLAM (Digital Subscriber Line Access Multiplexer), which is connected to the central splitter 205.

On the side of the subscriber 201, there is also a splitter 207 connected to the telephone line 200. The splitter 207 is called the master splitter. The master splitter 207 has the function of separating, on the subscriber side, the data signals, which are directed to a personal computer 208 associated with an ADSL modem, and the voice signals, which are directed towards conventional telephone handsets 209. Each telephone 209 connected to the same line 200 must be protected by a low-pass filter contained in the splitter 207. The compactness of these filters devoted them under the name micro-filters.

In theory, there is also, in each splitter 205 and 207, a high frequency filter, which passes only ADSL signals. In practice, the low frequency filter and the high frequency filter of the splitters are separate and distinct. The high frequency filter, that is to say the filter that passes the data signals and only those, is disposed on the telephone central side in the DSLAM 206, and subscriber side in a modem connected to the computer 208. The Low frequency filters are integrated into the splitters 205 and 207. Each low frequency filter serves to prevent sound signals from being disturbed by data signals. The object of the invention is the production of such a filter, having a very narrow transition band while being compact.

• le respect de valeurs d'impédances d'entrée et de sortie imposées. Afin que la puissance transmise par la ligne téléphonique le soit avec le meilleur rendement possible, les lignes téléphoniques doivent être terminées, en entrée et en sortie, par une impédance ayant une valeur aussi proche que possible de l'impédance caractéristique de la ligne téléphonique.
• le facteur dit d'affaiblissement d'insertion, ou ILF (pour Insertion Loss Factor en anglais), qui mesure un affaiblissement de puissance d'une voie de transmission lorsqu'il s'y trouve des montages en pont, des filtres, des égaliseurs ou d'autres montages. Dans le cas présent, ce facteur mesure la perte de transmission occasionnée dans le circuit par la mise en place d'un filtre, entre une situation où le filtre n'est pas présent et une situation où il est mis en place.
• le facteur dit d'affaiblissement de réflexion, ou RLF (pour Return Loss en anglais), qui est une mesure liée à la puissance de signal réfléchi par rapport à la puissance de signal transmissible. Lorsque l'adaptation est parfaite, le Return Loss tend vers l'infini.
• le facteur dit d'atténuation en bande ADSL, qui détermine une perte de puissance à faire subir aux signaux dont la fréquence appartient aux fréquences ADSL ;
• le facteur dit de distorsion d'insertion (insertion distortion en anglais), qui impose un variation maximale du facteur d'affaiblissement d'insertion.

The installation of filters, including low-pass filters, must meet several constraints dictated by different standards:

• the respect of input and output impedance values imposed. In order for the power transmitted by the telephone line to be as efficient as possible, the telephone lines must be terminated, at the input and at the output, by an impedance having a value as close as possible to the characteristic impedance of the telephone line.
• the insertion loss factor (ILF), which measures a power loss of a transmission path when there are bridge mounts, filters, equalizers or other montages. In the present case, this factor measures the transmission loss caused in the circuit by the setting up of a filter, between a situation where the filter is not present and a situation where it is put in place.
• the so-called reflection loss factor, or RLF (Return Loss in English), which is a measure related to the signal power reflected with respect to the transmittable signal power. When the adaptation is perfect, the Return Loss tends to infinity.
• the so-called attenuation factor in the ADSL band, which determines a loss of power to be subjected to the signals whose frequency belongs to the ADSL frequencies;
• the so-called distortion insertion factor (insertion distortion in English), which imposes a maximum variation of the insertion loss factor.

To try to satisfy all these constraints, it was imagined different types of low frequency filters. One of the preferred solutions of the state of the art, for producing low-pass filters intended to be arranged on telephone lines, is the use of elliptical filters, also called Cauer filters, known to those skilled in the art. .

These filters consist of a succession of elliptical cells, the basic structure of which is represented figure 3a . One of the strands of the telephone line L comprises, connected in parallel, an inductance L50 and a first capacitor C51 which constitute an anti-resonant circuit. A second capacitor C52 is disposed between the two strands of the line L. In practice, this basic cell is balanced to meet the standards of telephony. The left part of the figure 3-b shows an electronic circuit corresponding to an embodiment of the elliptical cell repeated in cascade, and the right part of the figure 5-b shows the dual shape of this electronic circuit. Such filters have, in particular, the characteristic of being able to present, by choosing appropriate component values, a large rejection ratio between the signals whose frequencies belong to the band of the sound signals and the signals whose frequencies belong to the band of the ADSL signals. .

However, such filter structures are not entirely satisfactory in practice from an economic point of view. Indeed, the requirements of the different standards in terms of ADSL signal rejection level and return loss imply the presence of at least two elliptical cells in cascade, and therefore a large number of inductors, within the low frequency filter. This has the consequence, on the one hand, of a non-negligible cost in the production of the filters, and, on the other hand, a space occupied by each splitter of the order of 15 to 20 cm ² . This size is not negligible, essentially the central telephone side where many splitters are mounted together on the same card.

One of the essential objects of the invention is to propose a low-pass filter which overcomes the disadvantages which have just been exposed. For this purpose, the low-pass filter according to the invention is an electronic circuit in which it is proposed to couple, within an autotransformer, different inductances present in the dual forms of the elliptic cells in cascade, and in particular in the non-linear forms. balanced of this electronic circuit, while compensating for a negative inductance induced by this coupling of inductances. In some embodiments of the invention, the compensation of this coupling takes the form of an active circuit emulating the characteristics of an inductance whose value is high enough to compensate for the induced negative inductance.

• un autotransformateur, constitué d'un enroulement d'une première inductance et d'un enroulement d'une deuxième inductance, bobinées autour d'un unique noyau magnétique, disposé entre un premier point d'entrée sur le premier brin de la ligne de transmission, un deuxième point d'entrée sur le premier brin de la ligne de transmission, et un troisième point d'entrée sur une première branche de dérivation reliant le premier brin et le deuxième brin;
• une première capacité disposée en série avec un troisième inductance sur la première branche de dérivation, la troisième inductance ayant une valeur au moins suffisante pour compenser une inductance négative induite par l'autotransformateur dans la première branche de dérivation.

The invention therefore essentially relates to a low-pass filtering device arranged on a telephone transmission line composed of a first strand and a second strand capable of running simultaneously between a telephone exchange and a subscriber equipped with least one telephone, first signals corresponding to analogue and / or digital narrow band services, and second signals corresponding to broadband services, characterized in that an unbalanced form of the filtering device comprises:

• an autotransformer, consisting of a winding of a first inductor and a winding of a second inductor, wound around a single magnetic core, arranged between a first point of entry on the first strand of the transmission line a second entry point on the first strand of the transmission line, and a third entry point on a first branch branch connecting the first strand and the second strand;
• a first capacitor arranged in series with a third inductor on the first branch branch, the third inductor having a value at least sufficient to compensate for a negative inductance induced by the autotransformer in the first branch branch.

The entire electronic circuit in the branch branch then forms a resonant circuit.

• la troisième inductance est réalisée sous la forme d'un premier circuit actif.
• la première inductance est disposée sur le premier brin de la ligne de transmission, et la deuxième inductance est disposée sur la première branche de dérivation, la première inductance et la deuxième inductance pouvant alors être enroulées autour de l'unique noyau magnétique de façon à réaliser un couplage avec renforcement négatif des inductances mutuelles.
• la première inductance et la deuxième inductance sont disposées sur le même premier brin de la ligne de transmission, la première inductance et la deuxième inductance pouvant alors être enroulées autour de l'unique noyau magnétique de façon à réaliser un couplage avec renforcement positif des inductances mutuelles. Cette caractéristique permet une réalisation du filtre passe-bas selon l'invention dans lequel un nombre plus faible de spires est nécessaire pour réaliser les inductances
• le dispositif de filtrage comporte :
  • une deuxième capacité, en série avec une quatrième inductance, disposés sur une deuxième branche de dérivation reliant le premier brin, au niveau du deuxième point d'entrée de l'autotransformateur, et le deuxième brin de la ligne de transmission ; ce circuit résonnant excédentaire crée alors un deuxième pôle de transmission;
  • une troisième capacité disposée sur une troisième banche de dérivation reliant le premier brin de la ligne de transmission, au niveau du deuxième point d'entrée de l'autotransformateur, au deuxième brin de la ligne de transmission.
  • la quatrième inductance est réalisée sous la forme d'un deuxième circuit actif ou passif.
  • le dispositif de filtrage peut être disposé du côté abonné ou du côté central téléphonique.

In addition to the main features just mentioned, the filtering device according to the invention may include one or more additional characteristics from the following:

• the third inductance is in the form of a first active circuit.
• the first inductor is arranged on the first strand of the transmission line, and the second inductor is arranged on the first branch branch, the first inductance and the second inductance can then be wound around the single magnetic core so as to achieve coupling with negative reinforcement of the mutual inductances.
• the first inductance and the second inductance are arranged on the same first strand of the transmission line, the first inductance and the second inductance then being able to be wound around the single magnetic core so as to carry out a coupling with positive reinforcement of the mutual inductances. . This characteristic allows an embodiment of the low-pass filter according to the invention in which a smaller number of turns is necessary to achieve the inductances.
• the filtering device comprises:
  • a second capacitance, in series with a fourth inductor, arranged on a second branch branch connecting the first strand, at the second point of entry of the autotransformer, and the second strand of the transmission line; this excess resonant circuit then creates a second transmission pole;
  • a third capacitor disposed on a third branch wall connecting the first strand of the transmission line, at the second point of entry of the autotransformer, to the second strand of the transmission line.
  • the fourth inductance is realized in the form of a second active or passive circuit.
  • the filtering device can be arranged on the subscriber side or the central telephone side.

Another object of the invention is the balanced shape of the device of low-pass filtering having the main characteristics and possibly one or more additional characteristics which have just been mentioned.

• à la figure 1, déjà décrite, une représentation de l'exploitation de la bande de fréquence en distribution ADSL ;
• à la figure 2, déjà décrite, une illustration du principe de la séparation des signaux basse fréquence et haute fréquence sur une ligne ADSL ;
• aux figures 3-a et 3-b, déjà décrites, différentes représentations de cellules elliptiques,
• à la figure 4, un exemple de filtre passe-bas utilisé dans l'état de la technique ;
• à la figure 5, une représentation non équilibrée d'un filtre passe-bas selon l'invention intégré dans le circuit électronique de la figure 4. ;
• aux figures 6-a et 6-b, un premier exemple de réalisation d'un autotransformateur intervenant dans les filtres passe-bas selon l'invention et un circuit électronique équivalent à cet autotransformateur;
• aux figures 7-a et 7-b, un deuxième exemple de réalisation d'un autotransformateur intervenant dans les filtres passe-bas selon l'invention et un circuit électronique équivalent à cet autotransformateur;
• à la figure 8, un exemple de réalisation d'un circuit actif susceptible d'intervenir dans le filtre passe-bas selon l'invention ;
• à la figure 9, une représentation non équilibrée d'une intégration d'un exemple de réalisation du filtre passe-bas selon l'invention dans un circuit électronique particulier ;
• à la figure 10, une représentation équilibrée de l'exemple de la figure 10.

The invention and its various applications will be better understood by reading the following description and examining the figures that accompany it. These are presented only as an indication and in no way limitative of the invention. The figures show:

• to the figure 1 , already described, a representation of the operation of the frequency band in ADSL distribution;
• to the figure 2 , already described, an illustration of the principle of the separation of low frequency and high frequency signals on an ADSL line;
• to the Figures 3-a and 3-b , already described, different representations of elliptical cells,
• to the figure 4 an example of a low-pass filter used in the state of the art;
• to the figure 5 , an unbalanced representation of a low-pass filter according to the invention integrated in the electronic circuit of the figure 4 . ;
• to the Figures 6-a and 6-b , a first embodiment of an autotransformer intervening in the low-pass filters according to the invention and an electronic circuit equivalent to this autotransformer;
• to the Figures 7-a and 7-b a second embodiment of an autotransformer intervening in the low-pass filters according to the invention and an electronic circuit equivalent to this autotransformer;
• to the figure 8 an embodiment of an active circuit capable of intervening in the low-pass filter according to the invention;
• to the figure 9 , an unbalanced representation of an integration of an exemplary embodiment of the low-pass filter according to the invention in a particular electronic circuit;
• to the figure 10 , a balanced representation of the example of the figure 10 .

The elements appearing in different figures will have kept the same references.

The figure 4 represents an example of a low-pass filter architecture 400 used in the state of the art for the filtering of the ADSL signals. The represented architecture corresponds to the unbalanced form of the filter 400; an unbalanced form of a filter intended to be arranged on a telephone line is always associated with a balanced shape in which the filter under consideration is symmetrized; in particular, each electronic component present on a first strand is then present on the second strand so that so-called common mode parasites affect the two line strands in the same way and can be differentially removed from the receiver. The values of the components of a balanced shape, arranged along the strands of the line, are, according to their capacitive or inductive nature, multiplied or halved with respect to the values of the corresponding components of the unbalanced form. The values of the components arranged between the two strands remain unchanged during the transition from the unbalanced form to the balanced form.

On the figure 4 a conventional telephone line L is composed of a first transmission line strand 401, called strand A or line TIP, and a second strand of transmission line 402, called strand B or line RING. The description of the figure representing the structure of the filter is from left to right, ie starting, for each transmission line, the line side (with a high impedance input for the DSL signals), leading to the central office. telephone 202, called the POTS side by the person skilled in the art (Plain Old Line Telephone System in English). It will be the same for the following figures where the telephone line L appears.

The filter 400 is composed, in their dual form, of a first elliptical cell 403 in cascade with a second elliptical cell 404. The first elliptical cell 403 is composed of a first inductor L1 disposed on the first strand of the transmission line. 401, and a second inductor L2 in series with a first capacitor C2 on a first branch branch 405 connecting the first strand 401 and the second strand 402. The second elliptical cell 404 is composed of a third inductor L3 disposed on the first strand of the transmission line 401, and a fourth inductance L4 in series with a second capacitor C4 on a second branch branch 406 connecting the first strand 401 and the second strand 402 of the transmission line. A third branch branch 407 completes the filter 400; it comprises a third capacitor C5, essentially intended to limit the noise at the output of the filter 400.

The different inductances present in the filter 400 are associated with distinct magnetic cores. One of the essential objects of the invention is to limit, in the low-pass filters used in the filtering of ADSL signals and whose circuit 400 is an example, the number of magnetic cores used. Thus, in the invention, the use of passive autotransformers is proposed which allow the grouping of several inductances around the same magnetic core. This general principle of the invention is illustrated in figure 5 , where an electronic circuit 500 is shown. The electronic circuit 500 corresponds to the low-pass filter 400 in which the group consisting of the three inductors L1, L2 and L3 has been replaced by a passive autotransformer 501, various embodiments of which will be detailed later. The autotransformer 501 has three input points: a first input point 502 on the first strand of the transmission line 401, a second input point 503 also on the first strand of the transmission line 401, and a third input point 502 on the first strand of the transmission line 401. input point 504 on the first branch branch 405. A compensation inductor L "2 has been added in the first branch branch 405 to compensate for an induced negative inductance generated by the autotransformer 501 as will be explained in FIG. following the description.

A first example 600 of passive autotransformer is shown in FIG. Figure 6-A . In this figure, it can be seen that the autotransformer 600 consists of the winding of a first inductance L'1, arranged on the first strand 401, and a second inductance L'2, disposed on the first branch of derivation 405, around the same magnetic core. The first inductor L'1 is connected between the first input point 502 and a connection point 601, located on the first strand 401, and directly connected to the second input point 503. The connection point 601 represents the point of contact. connection between the first inductor 1 and the second inductor 2 within the autotransformer 600. The second inductor 2 is connected between the third input point 504 and the connection point 601.

$$\mathrm{Lʹ}\mathrm{1}\mathrm{=}\mathrm{L}\mathrm{1}\mathrm{+}\mathrm{L}\mathrm{3}$$

$$\mathrm{Lʹ}\mathrm{2}\mathrm{=}{\mathrm{L}\mathrm{3}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{L}\mathrm{1}\mathrm{+}\mathrm{L}\mathrm{3}\mathrm{)}$$

In this example, the first inductor 1 and the second inductor 2 are coupled with a negative reinforcement of the mutual inductances. So, if we consider that M is the mutual inductance of the autotransformer 600, a circuit equivalent to the circuit of the Figure 6-A , without the presence of mutual inductance, is represented at Figure 6-B . This circuit comprises, as shown, a first value inductance L'1-M and a second inductance of value M on the first strand of the transmission line 401, and a third inductance, called inductance induced, of value 2- M on the first derivation branch 405. Comparing this architecture to that formed by the inductances L1, L2 and L3 of the figure 4 and assuming a perfect coupling, one easily reaches the following equations: $$\mathrm{M}\mathrm{=}\mathrm{The}\mathrm{3}$$

$$\mathrm{The}\mathrm{1}\mathrm{=}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}$$

$$\mathrm{The}\mathrm{2}\mathrm{=}{\mathrm{The}\mathrm{3}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{)}$$

Moreover, in the coupling carried out between L'1 and L'2, where L'1 is greater than L'2, it is shown that the inductance of value L'1-M is positive and that the induced inductance of value 2-M is negative. Thus, in order to compensate for this negative value of induced inductance, the inductance L "2 is introduced into the circuit according to the invention, which can be seen in particular in FIG. figure 5 who, so that the circuit of the figure 5 corresponds to that of the figure 4 , must respect the following equation: $$\mathrm{The}\mathrm{"}\mathrm{2}\mathrm{=}\mathrm{The}\mathrm{2}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{-}{\mathrm{The}\mathrm{3}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{)}$$

A second example 700 of passive autotransformer is shown in FIG. figure 7-A . In this figure, it can be seen that the autotransformer 700 consists of the winding of a first inductor L'1, arranged on the first strand of the transmission line 401, and a second inductance L'3, also arranged on the first strand 401, around the same magnetic core. The first inductor 1 is connected between the first input point 502 and a connection point 701, located on the first strand of the transmission line 401, and directly connected to the third input point 504. As in the first As shown, the connection point 701 represents the point of connection between the first inductance L1 and the second inductor L3 within the autotransformer 700. The second inductor L3 is connected between the second entry point 503 and the liaison point 701.

$$\mathrm{Lʹ}\mathrm{3}\mathrm{=}{\mathrm{L}\mathrm{3}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{L}\mathrm{1}\mathrm{+}\mathrm{L}\mathrm{3}\mathrm{)}$$

$$\mathrm{M}\mathrm{=}\mathrm{L}\mathrm{1}\mathrm{L}\mathrm{3}\mathrm{/}\left(\mathrm{L}\mathrm{1}\mathrm{+}\mathrm{L}\mathrm{3}\right)$$

In this example, the first inductor 1 and the second inductor 3 are coupled with a positive reinforcement of the inductances mutuals. Thus, if we consider that M is the mutual inductance of the autotransformer 700, a circuit equivalent to the circuit of the figure 7-A , without the presence of mutual inductance, is represented at figure 7-B . This circuit comprises, as shown, a first value inductance L'1 + M and a second inductance of value L'3 + M on the first strand 401, and a third inductance, called inductance induced, of value -M on the first branch 405. Comparing this architecture to that formed by the inductors L1, L2 and L3 of the figure 4 , we easily reach the following equations: $$\mathrm{The}\mathrm{1}\mathrm{=}{\mathrm{The}\mathrm{1}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{)}$$

$$\mathrm{The}\mathrm{3}\mathrm{=}{\mathrm{The}\mathrm{3}}^{\mathrm{2}}\mathrm{/}\mathrm{(}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{)}$$

$$\mathrm{M}\mathrm{=}\mathrm{The}\mathrm{1}\mathrm{The}\mathrm{3}\mathrm{/}\left(\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\right)$$

Moreover, in the coupling made between L'1 and L'3, the induced inductance of value -M is negative. Thus, in order to compensate for this negative value of induced inductance, the inductance L "2 is introduced into the circuit according to the invention, which can be seen in particular in FIG. figure 5 who, so that the circuit of the figure 5 corresponds to that of the figure 4 , must respect the following equation: $$\mathrm{The}\mathrm{"}\mathrm{2}\mathrm{=}\mathrm{The}\mathrm{2}\mathrm{+}\mathrm{The}\mathrm{1}\mathrm{The}\mathrm{3}\mathrm{/}\mathrm{(}\mathrm{The}\mathrm{1}\mathrm{+}\mathrm{The}\mathrm{3}\mathrm{)}$$

Thus, in the low-pass filters according to the invention, instead of a T-arrangement of three inductances attached to three separate magnetic cores, it is proposed, as is the case, for example, in FIG. figure 4 , the introduction of a passive autotransformer 600 or 700, consisting of two inductors wound around the same magnetic core, performing a coupling with positive or negative reinforcement, depending on the case, mutual inductances. It is furthermore provided to add a compensation inductor to at least compensate for a negative induced inductance generated in a bypass branch by the autotransformer. In practice, the compensation inductance L "2 may be small because the presence of the capacitor C2 in series with L" 2 ensures that the inductor is not traversed by any direct current.

In addition, it is advantageously provided in the low-pass filters according to the invention to replace the compensation inductance L "2 by an active circuit emulating this capacitance, this active circuit being of a simple design because it is not traversed by the Such an active circuit 800 is represented on the left part of the figure 8 . This embodiment is possible due to the absence of direct current in branch branch 405 due to the presence of a capacitance in this branch.

The active circuit 800 comprises in particular two operational transconductance amplifiers, or OTAs (for Operational Transonductance Amplifiers) which are voltage-controlled current sources, and which transform a voltage Vi into a current li of intensity equal to gmiVi, where gmi is the transconductance of the considered amplifier, and where i is a natural integer. Such a circuit makes it possible to replace a Li inductor by an active circuit integrating a capacitance Ci ', the active circuit can thus easily be made in the form of an ASIC. The relation between the capacitance Ci 'present in the active circuit 800 and the inductance LI that it replaces is given by the equation: $$\mathrm{Li}=\mathrm{This}/\left(\mathrm{gm}1\mathrm{gm}2\right)$$

The figure 9 shows a particular embodiment, in its unbalanced form, of a low-pass filter 900 according to the invention. With adequate values, such a filter has the same characteristics in terms of return loss, rejection loss, and attenuation as the larger circuit of the figure 4 . The illustrated circuit 900 corresponds, by way of example only, to the use of the autotransformer described in FIG. figure 7-A ; the autotransformer described at the Figure 6-A could also have been used. In the example illustrated, the inductance L4 has been replaced by an active circuit 901 similar to the active circuit 800, with a capacitance C4 'equal to L4 (gm3.gm4), according to the equation (3.1), and where gm3 and gm4 are the two transconductances of the two transconductance operational amplifiers involved in the production of the active circuit 901.

• L'1=5.97 mH
• L'3= 15.4 mH
• L"2= 9 8 mH
• L4= 1.15 mH
• C2= 68 nF
• C4= 33 nF
• C5= 1 n F

In the illustrated embodiment of low-pass filter embodiment according to the invention, the following values, to the nearest ten percent, for the different components of the unbalanced form are adopted to satisfy the different filtering constraints of the ADSL signals:

• The 1 = 5.97 mH
• 3 = 15.4 mH
• L "2 = 9 8 mH
• L4 = 1.15 mH
• C2 = 68 nF
• C4 = 33 nF
• C5 = 1n F

• L'1= 40.5 mH
• L'2= 15.4 mH
• L"2= 9.8 mH
• L4= 1.15 mH
• C2= 68 nF
• C4= 33 nF
• C5= 1 nF

In the non-illustrated embodiment of low pass filter according to the invention, corresponding to the use of the autotransformer of the Figure 6-A , to satisfy the various filtering constraints of the ADSL signals, the following values, to the nearest ten percent, are adopted for the different components of the unbalanced form:

• 1 = 40.5 mH
• 2 = 15.4 mH
• L "2 = 9.8 mH
• L4 = 1.15 mH
• C2 = 68 nF
• C4 = 33 nF
• C5 = 1 nF

For the realization of the balanced form, visible to the figure 10 , of the circuit represented in figure 9 , or for the realization of the balanced form of the circuit similar to that of the figure 9 but using the autotransformer of the Figure 6-A the values of the different inductances are, as previously mentioned, doubled and the values of the different capacitances are divided by 2 with respect to the two sets of values that have just been given.

Claims (13)

1. A low-pass filtering device placed on a telephone transmission line (L), composed of a first wire (401) and a second wire (402), capable of allowing the simultaneous circulation, between a telephone exchange (202) and a subscriber (201) equipped with at least one telephone, first signals, corresponding to analogical and/or digital narrow band services, and second signals, corresponding to broadband services, characterized in that an unbalanced shape of the filtering device (500) comprises:

   - an autotransformer (501), constituted of a first inductance (L'1) winding and a second inductance (L'2; L'3) winding, coiled around a single magnetic core, placed between a first input point (502) on the first transmission line wire (401), a second input point (503) on the first transmission line wire (401), and a third input point (504) on a first bypass branch (405) linking the first wire (401) and the second wire (402);

   - a first capacity (C2) placed in series with a third inductance (L"2) on the first bypass branch (405), the third inductance (L"2) having a value at least sufficient to compensate a negative inductance induced by the autotransformer (501) inside the first bypass branch (405).
2. A filtering device according to the previous claim, characterized in that the third inductance (L"2) is achieved in the form of a first active circuit (800).
3. A filtering device according to at least one of the previous claims, characterized in that the first inductance (L'1) is placed on the first transmission line wire (401) and that the second inductance (L'2) is placed on the first bypass branch (405).
4. A filtering device according to the previous claim, characterized in that the first inductance (L'1) and the second inductance (L'2) are coiled around the unique magnetic core in order to achieve a coupling with a negative reinforcement of the mutual inductances.
5. A filtering device according to at least one of the claims 1 or 2, characterized in that the first inductance (L'1) and the second inductance (L'3) are placed on the first bypass branch of the transmission (401).
6. A filtering device according to the previous claim, characterized in that the first inductance (L'1) and the second inductance (L'3) are coiled around the unique magnetic core in order to achieve a coupling with a positive reinforcement of the mutual inductances.
7. A filtering device according to at least one of the previous claims, characterized in that it comprises:

   - a second capacity (C4), in series with a fourth inductance (L4), placed on a second bypass branch (406) linking the first transmission line wire (401), at the level of the second input point (503) of the autotransformer (501), to the second transmission line wire (402);

   - a third capacity (C5) placed on a third bypass branch (406) linking the first transmission line wire (401), at the level of the second input point (503) of the autotransformer (501), to the second transmission line wire (402).
8. A filtering device according to the previous claim, characterized in that the fourth inductance (L4) is achieved in the form of a second active circuit (901).
9. A filtering device according to at least one of the previous claims, characterized in that it is placed on the side of the subscriber (201).
10. A filtering device according to at least one of the previous claims, characterized in that it is placed on the side of the telephone exchange (202).
11. A filtering device according to claim 7 and to at least one of the claims 1 to 4, characterized in that the values of the various components existing inside the filtering device are, with a 10% tolerance, the following:

    L'1= 40.5 mH

    L'2= 15.4 mH

    L"2= 9.8 mH

    L4= 1.15 mH

    C2= 68 nF

    C4= 33 nF

    C5=1 nF
12. A filtering device according to claim 7 and to at least one of the claims 1 to 2 or 5 to 6, characterized in that the values of the various components existing inside the filtering device are, with a 10% tolerance, the following:

    L'1= 5.97 mH

    L'2= 15.4mH

    L"2= 9.8 mH

    L4= 1.15 mH

    C2= 68 nF

    C4= 33 nF

    C5= 1 nF
13. A balanced form of the filtering device according to at least one of the previous claims.

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